This application is based on Patent Application No. 2001-30186 filed Feb. 6, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink-jet printing apparatus of a so-called serial scan type and a method for performing ink-jet printing using such an apparatus.
2. Description of the Related Art
An ink-jet printing apparatus prints an image on a printing medium such as a sheet of paper or a thin plastic sheet by ejecting ink from an ink-jet printing head onto the surface of the printing medium, fixing the ink, and imparting color thereto. A multi-color image can be obtained using a plurality of color inks, such as cyan (C), magenta (M), yellow (Y), and black (Bk). In this case, for example, a typical ink-jet printing head has a plurality of nozzle rows corresponding to their respective color inks, so that the multi-color image can be printed on the printing medium by ejecting ink for each of colors from the nozzle rows (hereinafter, also referred to as nozzle array) onto the printing medium as needed.
For preventing ink from blotting on the image printed on the printing medium (e.g., a sheet of paper), ink with a comparatively high permeability is often used as ink of each color except black. Also, ink with comparatively low permeability is often used as a black ink for the printing of a character image because it is capable of printing the edge of the character image sharply. If the black ink with a high permeability is used, on the other hand, the edge of the character becomes jagged. In other words, when the black ink touches on the surface of the paper, the black ink may move along fibers of the paper such that it sinks into the paper quickly compared with other color inks.
For an ink-jet printing system, from above point of view, it is usual to use an ink composition with comparatively low permeability as a black ink and an ink composition with comparatively high permeability as each of color inks. In such an ink-jet printing system, the blotting of ink may be caused on the boundary between a black image (i.e., an image formed by ink of black) and the color image (i.e., an image formed by at least one color ink except black) when the black ink and the other color inks make contact with each other on the printing medium. Hereinafter, such a blotting of ink will be referred to as xe2x80x9ca black-color blottingxe2x80x9d.
For avoiding or relieving the problem to be caused by the blotting of ink on the printing medium, as shown in FIG. 7, there is a method of arranging nozzle rows of a printing head in a serial scan type ink-jet printing apparatus. Such a printing apparatus sequentially performs the image formation on a printing medium by repeating a scanning of the printing head in the main-scan direction and a transfer of the printing medium in the sub-scan direction perpendicular to the main-scan direction. When the printing head is scanning, ink is ejected from nozzles arranged on the printing head.
Referring now to FIG. 7, there is shown an ink-jet printing head H provided with the ink-jet printing apparatus. The printing head H has several nozzle rows arranged in parallel on the surface thereof. Each nozzle row consists of a series of nozzles and extends in the direction Y for transferring a printing medium P (i.e., the sub-scan direction Y). These nozzle rows include: a nozzle row Bk for ejecting ink of black; a set of nozzle rows C1 and C2 for ejecting ink of cyan; a set of nozzle rows M1 and M2 for ejecting ink of magenta, and a set of nozzle rows Y1 and Y2 for ejecting ink of yellow. In the figure, the arrow X1 indicates the forward movement of the printing head H in the main-scan direction, while the arrow X2 indicates the backward movement thereof. In this conventional example, the printing head H is able to perform printing movements in both directions X1, X2 (i.e., bi-directional printing movement). If the printing head H scans in the forward direction X1, for example, it is allowed to perform the printing movement by ejecting ink from nozzle rows C1, M1, and Y1, respectively (i.e., the forward printing movement). If the printing head H scans in the backward direction X2, on the other hand, it is allowed to perform the printing movement by ejecting ink from nozzle rows C2, M2, and Y2, respectively (i.e., the backward printing movement). In these forward and backward printing movements, it is possible to eject black ink from the nozzle rows Bk as needed.
In the case of printing an image using only black ink, nozzles arranged on the entire length xe2x80x9cAxe2x80x9d of the black nozzle row Bk can be used. As shown in FIG. 7, the nozzle row Bk is designed to be longer than other nozzle rows C1, C2, M1, M2, Y1, and Y2, so that an area of the printing medium on which an image can be printed by the nozzle row Bk per one scanning movement of the printing head H (hereinafter, such an area is referred to as a printing area) can be larger than a printing area on which an image is printed by at least one of other nozzle rows C1, C2, M1, M2, Y1, and Y2. Therefore, a high-speed printing of a mono-color image only with black ink can be realized.
In the case of printing a multi-color image, a part xe2x80x9caxe2x80x9d of the nozzle row Bk is used. In this case, in addition, a part xe2x80x9cbxe2x80x9d (with a length equal to the part xe2x80x9caxe2x80x9d) of each of the nozzle rows C1, C2, M1, M2, Y1, and Y2 is also used. First, the printing medium P is moved to the position [1] and then the printing head H is moved in the direction X1 while ejecting black ink from the part xe2x80x9caxe2x80x9d of the nozzle row Bk depending on an image to be printed (hereinafter, referred to as a printing image). As a result, the image is printed on an area S1 on the printing medium P. Subsequently, the printing medium P is moved from the position [1] to the position [2]. In this case, the position [2] is apart from the position [1] at a distance corresponding to the length of the part xe2x80x9caxe2x80x9d in the sub-scan direction Y. Then, the printing head H is moved in the direction X2 while ejecting cyan ink, magenta ink, and yellow ink from the respective parts xe2x80x9cbxe2x80x9d of the nozzle rows C2, M2, and Y2 (hereinafter, these inks are collectively referred to as xe2x80x9ccolor inksxe2x80x9d) to complete the formation of a multi-color image on the area S1 on the printing medium P. During the period of moving the printing head H in the direction X2, an additional image formation is performed on a next area S2 on the printing medium P by ejecting black ink from the part xe2x80x9caxe2x80x9d of the nozzle row Bk depending on the printing image. Subsequently, the location of the printing medium P is shifted to the position [3] apart from the position [2] by the length of the part xe2x80x9caxe2x80x9d in the sub-scan direction Y. Then, the printing head H is moved in the direction X1 while ejecting color inks from the respective parts xe2x80x9cbxe2x80x9d of the nozzle rows C2, M2, and Y2 to complete the formation of a multi-color image on the area S2 on the printing medium P. During the period of moving the printing head H in the direction X1, an additional image formation is performed on a next area S3 on the printing medium P by ejecting black ink from the part xe2x80x9caxe2x80x9d of the nozzle row Bk depending on the printing image.
In such a printing method, the black ink is applied on the predetermined area of the printing medium P at first and then the color inks are applied on the same area during the next scanning movement of the printing head H. Therefore, a pass time between a performing time of the printing by the black ink and a performing time of the printing by the color inks is long. Consequently, it is preferable to prevent ink from blotting on the printing medium P, compared with the case of applying black and color inks on the printing medium P during the same scanning movement of the printing head H. In other words, the above method allows the decrease in the black-color blotting because the time required for the permeation and fixation of black ink into the printing medium P can be secured prior to applying color ink on the printing medium.
Furthermore, as shown in FIG. 8, for printing an image on a top-end portion of the printing medium P by the ink-jet printing apparatus, the distance between the printing medium P and the printing head H can be varied in a predetermined range. That is, the top-end portion of the printing medium P is located upstream between a transport roller R1 and a pinch roller R2 and is free of a discharge roller R3 and a discharge spur R4. Therefore, the distance between the printing medium P and the printing head H can be varied in a predetermined range according to a property of the printing medium P and a variation of application of ink on the printing medium P. Subsequently, as shown in FIG. 9, the variation in distance between the printing medium P and the printing head H decreases as the leading or top-end portion of the printing medium P becomes introduced between the discharge roller R3 and the discharge spur R4. In other words, the variation in distance between the printing medium P and the printing head H increases until the top-end portion of the printing medium P enters between the discharge roller R3 and the discharge spur R4. Then, the distance between the printing medium P and the printing head H varies when the top-end portion of the printing medium P enters between the discharge roller R3 and the discharge spur R4. Therefore, in the case of using the printing method as shown in FIG. 7, the printing position of an image can be shifted before and after intermittent transport of the printing medium P (hereinafter, also referred to as paper feed) when the image formation is performed on the top-end portion of the printing medium P.
For solving such a problem, there is a method in which the number of nozzles to be used for printing an image on a printing medium P is reduced until the top-end portion of the printing medium P enters between the discharge roller R3 and the discharge spur R4 or the length of the nozzle row to be used is previously shortened. In the method, the amount of intermittent transport of the printing medium P, i.e., the amount of paper feed is minimized on basis of the reduced nozzle number or the shorted nozzle row length. Such a method is able to complete the image formation with a comparatively small variation in distance between the printing medium P and the printing head H to relieve the problem in which the location of printing an image is shifted from the predetermined position.
Depending on the image data to be printed, there may be a case where a large amount of current that exceeds the capacity of a power supply is required for driving the printing head H. In such case, the amount of current to be required for driving the printing head H can be minimized by decreasing the number of nozzles to be used in the image formation or by shortening the length of each nozzle row to be used. In general, the number of data to be printed is counted every predetermined printing area, followed by determining whether there is a need for the control of decreasing the amount of current for driving the printing head H on the basis of the resulting count. If there is a need for the control, then the number of nozzles or the length of each nozzle row to be used in the image formation is restricted.
Accordingly, such a requirement restricts the number of nozzles or the length of each nozzle row to be used in the image formation.
In such a restriction, for example, a printing method shown in FIG. 10 may be applied. In the case of performing a normal printing movement by the printing method shown in FIG. 10, the same procedures as those of FIG. 7 described above may be performed. That is, the entire area A of the nozzle row Bk is used for the image formation only with black ink, or the part xe2x80x9caxe2x80x9d of the nozzle row Bk and the part xe2x80x9cbxe2x80x9d of each of the nozzle rows C1, C2, M1, M2, Y1, and Y2 are used for the multi-color image formation. If there is a need to restrict the number of nozzles to be used in the image formation, the part xe2x80x9caxe2x80x2xe2x80x9d of the nozzle row Bk and the part xe2x80x9cbxe2x80x2xe2x80x9d (with a length equal to the part xe2x80x9caxe2x80x2xe2x80x9d) of each of the nozzle rows C1, C2, M1, M2, Y1, and Y2 are used.
In FIG. 10, just as in the case of FIG. 7, there are shaded portions. The shaded portion with lines slanting downward from right to left is an area on which an image can be printed using the nozzle row Bk. On the other hand, the shaded portion with lines slanting downward from left to right is an area on which an image can be printed using the nozzle rows C1, C2, M1, M2, Y1, and Y2. Therefore, the shaded portion with lines slanting downward from right to left and slanting downward from left to right is an area on which the image is printed using the nozzle rows C1, C2, M1, M2, Y1, and Y2 after the image is printed using the nozzle row Bk.
Referring again to FIG. 10, the printing movement will be explained. The printing movement is under the conditions in which the number of nozzles to be used for the printing is restricted. If the printing medium P is transferred to the position [1], just as in the case of FIG. 7, then the image formation is performed on both areas S1, S2 on the printing medium P by ejecting black ink from the part xe2x80x9caxe2x80x9d of the nozzle row Bk on the basis of image data to be printed. Here, if there is a need for restricting the number of nozzles or the length of each nozzle row to be used in the image formation due to the same fact as described above, the image is printed using the part xe2x80x9caxe2x80x2xe2x80x9d, which is one-half of the part xe2x80x9caxe2x80x9d (i.e., axe2x80x2=a/2) of the nozzle row Bk and the part xe2x80x9cbxe2x80x2xe2x80x9d, which is one-half of the part xe2x80x9cbxe2x80x9d (i.e., bxe2x80x2=b/2) of each of the nozzle rows C1, C2, M1, M2, Y1, and Y2. That is, the printing medium P is transferred to the position [2] and then color ink is ejected from the part xe2x80x9cbxe2x80x2xe2x80x9d of each of the nozzle rows C2, M2, and Y2 on the basis of the printing image to complete a multi-color image formation on the area S1 of the printing medium P. At this time, black ink is ejected from the part xe2x80x9caxe2x80x2xe2x80x9d of the nozzle row Bk on the basis of the printing image, so that an image can be printed on the next area S3 on the printing medium P. Subsequently, the printing medium P is transferred to the position [3] at a distance of the length of the part xe2x80x9caxe2x80x2xe2x80x9d from the position [2] in the sub-scan direction Y. Then, color ink is ejected from the part xe2x80x9cbxe2x80x2xe2x80x9d of each of the nozzle rows C1, M1, and Y1 on the basis of the printing image to complete a multi-color image formation on the area S2 of the printing medium P. At this time, black ink is ejected from the part xe2x80x9caxe2x80x2xe2x80x9d of the nozzle row Bk on the basis of the printing image, so that an image can be printed on the next area S4 on the printing medium P.
In the case shown in FIG. 10, therefore, if there is a need to restrict the number of nozzles or the length of the nozzle row to be used in the printing, the image formation may be performed using one-half of the nozzles generally used in the printing and reducing the amount of paper feed by half.
If there is a need to restrict the number of nozzles or the length of the nozzle row to be used in the printing, alternatively, another printing method may be applied as shown in FIG. 11. For performing the normal printing operation by the printing method shown in FIG. 11, just as in the case of FIG. 7 described above, the entire area xe2x80x9cAxe2x80x9d of the nozzle row Bk is used when the image formation requires only black ink, while the part xe2x80x9caxe2x80x9d of the nozzle row Bk and the part xe2x80x9cbxe2x80x9d (with a length equal to the part xe2x80x9caxe2x80x9d) of each of nozzle rows C1, C2, M1, M2, Y1, and Y2 is used for printing a multi-color image. If there is a need to restrict the number of nozzles, the part xe2x80x9caxe2x80x9d of the nozzle row Bk is divided into an upper half portion xe2x80x9ca1xe2x80x9d and a lower half portion xe2x80x9ca2xe2x80x9d and the part xe2x80x9cbxe2x80x9d of each of nozzle rows C1, C2, M1, M2, Y1, and Y2 is also divided into an upper half portion xe2x80x9cb1xe2x80x9d and a lower half portion xe2x80x9cb2xe2x80x9d so that these half portions can be used independently.
In the state [1], just as in the case with FIG. 7, the images are formed on the areas S1, S2 on the printing medium P respectively by ejecting black ink from the part xe2x80x9caxe2x80x9d of the nozzle row Bk depending on the printing image. Subsequently, the printing medium P is moved by the length of the part xe2x80x9caxe2x80x9d of the nozzle row Bk, followed by ejecting color inks from the upper portion xe2x80x9cb1xe2x80x9d of each of nozzle rows C2, M2, and Y2 onto the area S1 of the printing medium P depending on the printing image in the state [2] to complete the formation of a multi-color image on that area S1. At this time, black ink is ejected from the upper portion xe2x80x9ca1xe2x80x9d of the nozzle row Bk onto the next area S3 of the printing medium P depending on the printing image, so that an image can be printed on that area S3 on the printing medium P. After that, in the state [3], color inks are ejected from the lower portion xe2x80x9cb2xe2x80x9d of each of nozzle rows C1, M1, and Y1 onto the area S2 of the printing medium P depending on the printing image to complete the formation of a multi-color image on that area S2, without feeding the printing medium P. At this time, black ink is ejected from the lower portion xe2x80x9ca2xe2x80x9d of the nozzle row Bk onto the next area S4 of the printing medium P depending on the printing image, so that an image can be printed on that area S4 on the printing medium P. Subsequently, the printing medium P is fed by the length of the part xe2x80x9caxe2x80x9d of the nozzle row Bk. In the states [4] and [5], the same printing movements as those performed in the states [2] and [3] are repeated.
However, if there is a need to restrict the number of nozzles or nozzle rows to be used in the printing, in the case of using the printing methods shown in FIG. 10 and FIG. 11, there is a difference between the printing movement under the restriction and the normal printing movement under no restriction with respect to the timing of printing with a nozzle row for ejecting black ink, paper feed, and printing with nozzle rows for ejecting color inks.
In other words, in the normal printing movement, the formation of a printing image on an arbitrary printing area of the printing medium P can be completed by sequentially performing the steps of printing with the nozzle row for ejecting black ink, feeding the printing medium P, and printing with the nozzle rows for ejecting color inks. On the other hand, in the printing movement in which the number of nozzles or the length of the nozzle rows is restricted, in the case of the printing method shown in FIG. 10, the formation of a printing image on an arbitrary printing area of the printing medium P can be completed by sequentially performing the steps of printing with the nozzle row for ejecting black ink, feeding the printing medium P two times, and printing with the nozzle rows for ejecting color inks. In the case of the printing method shown in FIG. 11, furthermore, the formation of a printing image on an arbitrary printing area can be completed by sequentially performing the steps of printing with the nozzle row for ejecting black ink, feeding the printing medium, waiting for the printing movement (at this time, the printing movement on another printing area using nozzle rows for ejecting color inks is performed), and printing with the nozzle rows for ejecting color inks. Alternatively, the formation of such a printing image may be completed by sequentially performing the steps of printing with the nozzle row for ejecting black ink, waiting for the printing movement (at this time, the printing movement on another printing area using nozzle rows for ejecting color inks is performed), feeding the printing medium, and printing with the nozzle rows for ejecting color inks. In each of the printing methods shown in FIG. 10 and FIG. 11, therefore, when the number of nozzles or the length of the nozzle rows is restricted, an area printed by using the nozzle row for ejecting black ink is not printed by using the nozzle rows for ejecting color inks by the next printing scan. That area printed by using the nozzle row for ejecting black ink is printed by using the nozzle rows for ejecting color inks by the subsequent printing scan after that next printing scan to complete the image formation.
Consequently, there is a problem in which the printing quality of an image can be decreased when the printing movement with the restriction on the number of nozzles or the length of the nozzle rows and the printing movement without such a restriction are different from each other. In other words, they are different with respect to the degrees of permeation and fixation of black ink previously placed on the predetermined printing area of the printing medium when color inks are subsequently placed on that area. Therefore, there is a possibility of causing some trouble in the resulting image, such as undesired image-printing variations (e.g., inconsistencies in image density), by the difference in ink colors that come out or are formed on the printing medium.
It is an object of the present invention to provide a method and apparatus for ink-jet printing, which allows the printing of a high-quality image by keeping the conditions for printing movement to avoid variations in ink colors coming out on the printing medium regardless of the change in the number of nozzles to be used in the printing of an image and also regardless of the change in the amount of transferring the printing medium.
In the first aspect of the present invention, there is provided an ink-jet printing apparatus using a printing head having a plurality of nozzle rows each arranging nozzles for ejecting ink, to print an image on a printing medium by repeating the steps of ejecting ink from the nozzle rows of the printing head onto the printing medium while moving the printing head in a main-scan direction and transferring the printing medium in a sub-scan direction perpendicular to the main-scan direction, wherein:
at least one of the plurality of the nozzle rows of the printing head is arranged on a position displaced from other nozzle rows in the sub-scan direction, the ink-jet printing apparatus comprising:
changing means capable of changing the amount of transferring the printing medium; and
control means for restricting the number of nozzles to be used in the printing, which belong to the nozzle row located on an upstream side in the sub-scan direction, before changing the amount of transferring the printing medium by the changing means, and restricting the number of nozzles to be used in the printing, which belong to the nozzle row located on a downstream side in the sub-scan direction, after changing the amount of transferring the printing medium by the changing means.
In the second aspect of the present invention, there is provided an ink-jet printing method using a printing head having a plurality of nozzle rows each arranging nozzles for ejecting ink, to print an image on a printing medium by repeating the steps of ejecting ink from the nozzle rows of the printing head onto the printing medium while moving the printing head in a main-scan direction and transferring the printing medium in a sub-scan direction perpendicular to the main-scan direction, wherein:
at least one of the plurality of the nozzle rows of the printing head is arranged on a position displaced from other nozzle rows in the sub-scan direction, the ink-jet printing method comprising the steps of:
restricting the number of nozzles to be used in the printing, which belong to the nozzle row located on an upstream side in the sub-scan direction before changing the amount of transferring the printing medium; and
restricting the number of nozzles to be used in the printing, which belong to the nozzle row located on a downstream side in the sub-scan direction, after changing the amount of transferring the printing medium.
According to the present invention in one preferred mode, the number of nozzles to be used in the printing, belonging to the nozzle rows positioned on the upstream in the sub-scan direction, is restricted prior to change of the amount of transferring the printing medium. In addition, the number of nozzles to be used in the printing, belonging to the nozzle rows positioned downstream in the sub-scan direction, is restricted after the change the amount of transferring the printing medium. Consequently, the same printing conditions as those of the normal printing movement can be kept regardless of the change in the amount of transferring the printing medium and also regardless of the change in the number of nozzles to be used in the printing of an image. As a result, the printing of a high-quality image can be performed without variations in ink colors coming out on the printing medium.